1. Field of the Invention
The present invention relates to a photodetector for detecting optical information from a surface to be measured, and more particularly to improvements in a photodetector having a form for guiding light from a light emitting portion to a surface to be measured through a light guiding member and for guiding the detected light from the surface to be measured to a light receiving portion through the light guiding member, as well as a method of using the same, and an image forming apparatus using the same.
2. Description of the Related Art
Generally, photodetectors are arranged to detect optical information from a surface to be measured by applying light to the surface to be measured, and has been widely used in various fields.
For example, in an image forming apparatus such as a copying machine, a printer, and a facsimile equipment, adopting the electrophotographic system, a system is adopted in which a toner patch for density detection is formed on an image carrying body such as a photoconductor or an intermediate transfer member, and the photodetector (here, a toner amount detector) detects the amount of toner of the toner patch to perform image density control.
Here, a description will be given of general toner amount detectors, and there are the following two systems.
The first system is called a reflected-light detection system, and utilizes the fact that the surface of the image carrying body such as the photoconductor or an intermediate transfer member to which the toner adheres has a mirror surface structure having a high degree of flatness.
Namely, as shown in FIG. 19A, in this reflected-light detection system, light of a fixed intensity is applied from a light source 201 to a surface of an image carrying body 200, a photodiode 202, i.e., a light receiving element receives specularly reflected light thereof to convert the received light into a voltage corresponding to its intensity (see JP-A-6-175501).
At this time, the reflected light scatters at portions where a toner 203, which is a surface to be measured, adheres, and the quantity of light declines by that portion. Therefore, the output voltage drops at the portion where the quantity of light declines.
Consequently, relationship between the amount of toner and the output voltage is obtained as shown in FIG. 19B. The toner amount detection in the image forming apparatus becomes possible.
However, with this reflected-light detection system, if the surface of the image carrying body is substantially covered with the toner, it becomes impossible to obtain reflected light, rendering the detection impossible. Accordingly, with this reflected-light detection system, the toner amount detection is possible in an area where the toner adheres to the image carrying body by a very small amount or by about one layer.
In addition, the second system is called a scattered-light detection system, and is used in the case of a color toner, for example.
Namely, as shown in FIG. 20A, this scattered-light detection system utilizes the characteristic that if the light from a light source 211 is applied to a color toner 213 on the image carrying body 200, scattered light occurs due to the surface reflection and internal reflection. A photodiode 212, i.e., a light receiving element, is disposed at a portion other than a light receiving portion for receiving the specularly reflected light, and the aforementioned scattered light is monitored by this photodiode 212.
In the case of this system, as shown in FIG. 20B, as the amount of toner increases, the output voltage of the photodiode 212 changes in such a manner as to rise, thereby permitting the toner amount detection.
In this scattered-light detection system, even in a case where a plurality of layers of toner adhere to the image carrying body 200, the scattered light from the lower layer toner transmits through the surface layer toner and returns. For this reason, this scattered-light detection system can obtain an output even with respect to a greater amount of toner than that in the reflected-light detection system.
However, the scattered-light detected system cannot be applied to a black toner, which does not cause scattered light.
Each of the toner amount detectors (photodetectors) of these two systems includes a light source, lenses, a light receiving element, a drive circuit board, and the like. These constituent members are frequently arranged integrally and are disposed in the vicinity of a measuring point.
For this reason, the toner amount detector is generally disposed at a position distant from the measuring point by several millimeters to 10-odd millimeters and with a thickness and a width ranging from about 5 mm to about 15 mm.
At this time, the principal factor for restraining the size of the sensor (toner amount detector) is derived from need to dispose the optical system, the elements, and circuits concerning the measurement in the vicinity of the measuring point.
In view of the requirement of the illuminating optical system, if the distance between the light source and the light emitting element (light source) is made long, electromagnetic noise becomes liable to affect the system, making it difficult to ensure stability in the quantity of illuminating light.
In addition, in view of the requirement of the receiving optical system, an extremely weak current generated by the light receiving element needs to be amplified at a position as close to the light receiving element as possible so as to obtain a stable analog signal. For this reason, the light receiving element is often directly connected to the drive circuit board in the vicinity of the measurement position.
Further, the light emitting element and the light receiving element have millimeter of externals shapes, with the result that the sensor is provided with the above-described size.
Since this type of toner amount detector (sensor) is disposed in the vicinity of the measuring point, it is naturally necessary to secure an installation space for the sensor in the vicinity of the measuring point.
In recent years, however, in the light of demand for making the image forming apparatuses compact, various sub units tend to be arranged with high densities inside the image forming apparatuses.
For example, so-called tandem-type image forming engines in which four image forming units are arranged for forming toner images of various color components are becoming a mainstream.
In the case of such an apparatus, it is often impossible to obtain a space, which allows the sensor to be disposed in the vicinity of the position subject to measurement.
Since the amount of toner to be essentially detected cannot be detected in this case, the actual situation is such that it is inevitable to adopt a technique whereby process control is performed after the amount of toner is estimated from a substitute value.
In contrast, JP-A-7-177091 shows that the communication of main digital signals in an image forming apparatus including a toner amount detector and the like is effected by using optical fibers.
If this system is used, it is possible to alleviate the effect due to the electromagnetic noise of the signal.
If such a conventional technique is used, it is proposed that optical fibers are used to propagate an analog quantity of light, which the toner amount detector handles, as shown in JP-A-2002-98637, for example.
For example, as shown in FIG. 21, light from a light source 221 is led to a measuring point P by using an incident-side optical fiber 222. Detected light from the measuring point P is led to an output-side optical fiber 224 through a focusing lens 223 and is led to a light receiving element 225 such as a photodiode by using this output-side optical fiber 224.
In accordance with this form, it theoretically becomes possible to dispose the light source 221 and the measuring point P as well as the measuring point P and the light receiving element 225 in such a manner as to be spaced apart from each other.
In addition, only the optical fibers 222 and 224 and the focusing lens 223 are disposed in the vicinity of the measuring point P. Therefore, it becomes possible to realize a substantially compact sensor (toner amount detector) correspondingly.
Accordingly, even if the installation space in the vicinity of the measuring point P is narrow, it becomes possible to dispose the sensor efficiently.
However, with this type of toner amount detector, the following technical problems have been encountered.
First, with the above-described toner amount detector, the detected light from a spot-like measuring point P must be focused. Therefore, if the surface to be measured, which constitutes the measuring point P, becomes deteriorated, not only is the deterioration of the surface to be measured liable to immediately affect the detection accuracy, but the detected light from the measuring point P is difficult to be reliably focused onto the output-side optical fiber 224. At any rate, there is a technical problem in that the deterioration of the surface to be measured is liable to result in a decline in the detection accuracy.
In addition, although this type of toner amount detector uses optical fibers, the transmittance of the optical fiber is likely to vary due to a change in its shape and the like. For this reason, it is difficult to propagate the analog quantity of light stably.
Here, it is possible to cite the following as factors for the variation of the transmittance.                due to thermal expansion and a change in shape        due to contamination at a junction        a decline in the transmittance over time        
The effect of the variation of the transmittance on the detection accuracy will be examined. If it is now assumed that monitoring is performed in accordance with the reflected-light detection system by using the toner amount detector shown in FIG. 21, results shown in FIG. 22 are obtained.
At this time, if it is assumed that the transmittance has declined by, for example, 10%, it can be understood that the amount of toner measured which should be, for example, 0.095 mg/cm2 is erroneously measured to be 0.075 mg/cm2.
In such a situation, it is assumed that the measurement accuracy required is assumed to be about 0.01 mg/cm2. This portion alone of the variation of the quantity of light produces an amount of fluctuation, which is twice as large as a target value, causing a decline in the detection accuracy.